Molecular dynamics study of Eu³⁺ in an aqueous solution: Luminescence spectrum from simulated environments

Abstract

Because doping ions are mainly surrounded by solvent molecules in the early stages of sol—gel processing, the luminescence of rare-earth ions in such an environment is known to be very similar to that observed in aqueous solutions. Thus we performed a molecular dynamics simulation of the trivalent europium in aqueous solution in order to produce a statistical set of environments and to calculate the spectroscopic features thereof. From radial and angular ordering studies, the hydration number of Eu³⁺ was found to be between eight and nine. The eight-hydrated configurations are nevertheless predominant and present a specific arrangement, according to the geometry of a square antiprism polyhedron. From the whole set of simulated configurations, we computed the ligand field parameters without considering any a priori symmetry. We then calculated the energy splitting of each J manifold and the transition probabilities from the ⁵D₀ emitting state. A simulated luminescent spectrum was finally obtained and compared with the experimental sectrum. In addition to overall good agreement, we were able to analyse the ⁵D₀ → ⁷F₁ transition shape, which revealed a sensitive dependence on the local structure, and especially on the hydration number.